Conventionally, an electronic device is provided with a detachable adapter device to, e.g., expand and change a function. For example, as indicated in Patent Document 1, in a video camera device having a dockable structure, a camera main body unit and an adapter unit (a camera adapter, a videotape recorder, various other kinds of peripheral devices) are detachable in terms of electrical and mechanical manner at the front and the back in the optical axis direction.